1. Field of the Invention
The present invention relates to an electrical device having a PTC (Positive Temperature Coefficient) conductive polymer, and more particularly to a PTC electrical device, which is light, thin, short and small with giving excellent binding force between a conductive polymer and an electrode and not causing a breakdown during being combined.
2. Description of the Related Art
Electrical devices having PTC conductive polymer are well known in the art. Conductive polymer contains organic polymer in which conductive fillers are dispersed, and shows PTC characteristic. PTC characteristic means a property that electrical resistance is abruptly increased in a narrow temperature region, and polymer materials having such PTC characteristic are generally applied to a self-regulating heating cable, a protection device for blocking over current, a circuit protection element, a heater and so on.
Such conductive polymer is mechanically and chemically combined with at least one electrode in an electrical device. A metal plate is generally used as the electrode. Examples of such devices are disclosed in U.S. Pat. No. 4,426,633 by Taylor, U.S. Pat. No. 4,689,475 by Matthiesen, U.S. Pat. No. 4,800,253 by Kleiner et al., U.S. Pat. No. 4,857,880 by Au et al., U.S. Pat. No. 4,907,340 by Fang et al, and U.S. Pat. No. 4,426,633 by Fang et al.
The binding force between the metal plate and the conductive polymer may be generally classified into mechanical binding force and chemical binding force. For improving the mechanical binding force, there is needed a process of increasing surface roughness of the metal plate in order to restrain separation of the metal plate and the conductive polymer.
U.S. Pat. No. 4,689,475 and U.S. Pat. No. 4,800,253 uses a metal plate having a microrough surface as an electrode in order to increase binding force with conductive polymer. In particular, U.S. Pat. No. 4,689,475 limits height and width of irregularities formed on the surface of the electrode to suitable sizes in order to increase the binding force with the conductive polymer. In addition, U.S. Pat. No. 4,800,253 uses a metal plate having a microrough surface including macronodules formed by a plurality of micronodules, as an electrode.
Meanwhile, U.S. Pat. No. 5,874,885 uses a metal plate having a base layer, an intermediate layer and a surface layer as an electrode which is to be contacted with the conductive polymer.
Recently, as electronic equipments become lighter, thinner, shorter and smaller, the size of PTC element is also more reduced. Thus, the thickness of the conductive polymer sheet interposed between electrodes is required to be smaller in order to reduce the size of PTC element. If the thickness of the conductive polymer sheet is decreased, protrusions such as nodules or irregularities formed on the surfaces of the opposite electrodes are approached or contacted, which is apt to cause a breakdown.
In addition, if nodules or irregularities are set to be too small as the thickness of the conductive polymer sheet is reduced, the mechanical binding force between electrode and conductive polymer is deteriorated.
Thus, it is required to suitably control height, width and gap of nodules or irregularities formed on the surface of the electrode as well as the thickness of a conductive polymer suitable for PTC element applied to light, thin, short and small electronic equipments.
However, any document mentioned above does not suggest optimal values of height, width and gap of the protrusions for causing no breakdown and ensuring easy and sufficient adhesion to a relatively thin conductive polymer without air gap.